The present invention relates to an improved method for electric field poling of ferroelectric materials and apparatus for carrying out such a method.
Nonlinear optical devices are proving very useful in the field of optics for converting laser light between different wavelengths. Examples of conversion include second harmonic generation in which two photons combine to create a new photon at twice the frequency (half the wavelength), and optical parametric generation in which a single photon splits into two photons at longer wavelengths. In general, such processes must conserve photon energy and must be phase-matched to achieve useful efficiencies. The requirement for phase-matching is that the phase velocities of the interacting waves in the nonlinear material must be equal. One phase-matching technique makes use of birefringence in a crystalline material to achieve efficient interaction. An alternative technique is based on the concept of quasi-phasematching (QPM) in which the difference in phase velocities of the interacting waves is compensated by a periodic reversal of the nonlinear coefficient of the crystal. This periodic reversal can be achieved by many techniques, the most common of which uses an approach known as periodic poling, in which a high voltage pulse is applied to a ferroelectric material using a patterned electrode to create a corresponding pattern of domain reversals.
Many published periodic poling methods are based on that described in U.S. Pat. No. 5,193,023. This method uses a pair of electrodes formed on opposite major surfaces of a ferroelectric substrate, one of which is patterned according to the desired domain reversal pattern, for application of a DC voltage across the substrate to create a pattern of or opposite polarisation direction. To produce domain inversion, the voltage must exceed the so-called coercive voltage for the ferroelectric material being used. Furthermore, for this method, the voltage is to be a pre-determined DC voltage or predetermined voltage pulse.
To implement this method, it is necessary to know in advance the voltage that both produces poling but prevents catastrophic breakdown. This can be difficult, because the electrical and physical characteristics of the actual crystal in use are often not known well so there is a significant element of guess work in determining the voltage characteristics. Furthermore, when working with samples cut from a larger wafer, the properties may vary from sample to sample as composition or thickness varies across the wafer, making it necessary to change the voltage for every sample.
The method is widely used, however, and it is common practise to set up such voltage controlled poling by using a voltage programmed source and a large resistor in series with the crystal to be poled. A voltage somewhat larger than the coercive voltage is applied, with the resistor acting to limit the current. However, this technique results in limitation as the large resistor sets up a large time constant (together with the capacitance of the sample). Thus precise control is difficult to achieve, and the poling proceeds at a rate set by the coercive field of the sample, the series resistor and the programmed high voltage.
A later-developed poling technique uses control of current rather than voltage. The poling apparatus uses feedback to give a desired current shape by dynamic variation of voltage, which overcomes many drawbacks of the approach taught in U.S. Pat. No. 5,193,023 as it no longer requires a guess to be made as to the best poling voltage. Furthermore, it allows considerable control of the poling dynamic. Domain switching causes a displacement current which equals the current in the external poling circuit; thus the technique controls the rate of domain switching directly. The method also helps to recover from electrical breakdown events by limiting voltage to prevent high currents, in turn leading to improved yield.
Current-controlled poling has been described in many publications, for example U.S. Pat. No. 6,952,307. However, while disadvantages of the original voltage-controlled poling method are overcome, the approach of merely controlling the current does not necessarily lead to good results.